1. Field of the Invention
The present invention relates to a current detection circuit for an air/fuel ratio sensor (A/F sensor) used for detecting an air/fuel ratio in an internal combustion engine such as an automotive engine.
2. Prior Art
An A/F sensor is a sensor that monitors the air-to-fuel ratio (A/F ratio), i.e., A/F value, in an exhaust gas. This sensor is used to detect the ratio of air to fuel in the exhaust gas such as automotive exhaust, and to adjust, based on the detected value, the amount of fuel to be supplied to the engine to an optimum value. FIG. 1 shows the relationship between sensor current and A/F ratio in a conventional A/F sensor. The A/F sensor characteristic shown here was measured while applying a voltage of 0.4 V across the sensor. Further, the circuit was adjusted such that the sensor current was zero at stoichiometric point and was negative in the rich range and positive in the lean range.
As shown, the sensor current is substantially proportional to the A/F value, which means that the A/F value can be obtained by detecting the sensor current on the circuit. Further, to obtain the sensor characteristic, the A/F sensor must be activated and, for this purpose, the sensor element is usually preheated to a temperature of 550° C. or higher. The sensor's admittance-temperature characteristic shown in FIG. 2 is used to detect the temperature of the sensor element. Since the admittance-temperature characteristic is unique to each particular sensor, the temperature T1 of the element at a given instant in time can be detected by measuring the element's admittance value Ad1 at that instant and comparing the measured value against a premeasured admittance-temperature characteristic curve. Here, the admittance Ad is the reciprocal of the impedance Z.
To summarize, the A/F sensor processing circuit is required to have two functions: the function (function 1) of monitoring the sensor current in order to obtain the A/F value, and the function (function 2) of detecting the sensor's admittance value in order to obtain the temperature of the sensor element.
FIG. 3 shows a prior art sensor current detection circuit equipped with the above two functions. In the figure, reference numeral 1 is a two-terminal A/F sensor having a positive terminal 2 and a negative terminal 3; as shown, this sensor can be represented in the form of an equivalent circuit as a series circuit of impedance Z and electromotive force E. The two terminals 2 and 3 are connected to an A/F sensor circuit 4 mounted in an ECU (Electronic Control Unit). As shown, the A/F sensor circuit 4 comprises operational amplifiers 5 and 6 and shunt resistors Ra and Rb.
The output terminal of the operational amplifier 5 is connected via the shunt resistor Ra to the positive terminal 2 of the sensor 1. The positive terminal 2 of the sensor 1 is in turn connected to the positive input terminal of the operational amplifier 5. The negative input terminal of the operational amplifier 5 is connected to a variable voltage power supply 7 of a variable voltage V1, while the negative input terminal of the operational amplifier 6 is connected to a fixed voltage power supply of a voltage V2. The operational amplifiers 5 and 6 operate so as to make the voltage values at the positive and negative terminals 2 and 3 of the sensor 1 equal to the voltage V1 of the variable voltage power supply 7 and the voltage V2 of the fixed voltage power supply 8. Each of the operational amplifiers 5 and 6 is connected between a +B power supply and ground.
Reference numeral 9 is an AD converter having terminals AD1 and AD2 for monitoring the voltage across the shunt resistor Ra. The voltage value detected by the converter 9 is sent to a microcomputer not shown, where prescribed processing is performed to compute the sensor current and the admittance value.
The operation of the above sensor current detection circuit 4 will be described below to clarify the drawback of the prior art. First, for simplicity of explanation, it is assumed that the allowable voltage range of the AD converter 9 is from 0 V to 5 V, and that the voltages V1 and V2 are set to 3.3 V and 2.9 V, respectively, for detection of the sensor current.
In the above circuit, the function 1 is carried out by measuring the sensor current=(AD2−AD1)/R while holding the voltage V1 at a constant level (for example, 3.3 V). On the other hand, the function 2 can be accomplished by obtaining the admittance from the amount of change of the current (ΔI) when the voltage V1 is varied (ΔV). The admittance Z of the sensor 1 in this case is given by the equationAdmittance=ΔI/ΔV  (1)
Actual voltage sweeping waveforms V1 and V2 are shown in FIG. 4.
The waveform V1 is formed by alternating a period b, during which the voltage is swept with ΔV=0.2 V, and a constant voltage period a, during which the voltage is held constant, the combined length of the periods a and b being a few hundred milliseconds. The element's admittance can be obtained from the above equation (1) by obtaining the amount of change of the current, ΔI, due to the impedance within the sensor during the voltage sweeping period b, as described above. At this time, the voltage detected by the terminal AD2 of the AD converter 9 rises up to the “B” voltage in terms of a peak value.
On the other hand, the sensor current monitoring function 1 is carried out during the period a where the voltage V1 is held fixed to 3.3 V. At this time, the voltage at the terminal AD2 of the AD converter 9 remains fixed to “A” voltage. As an example, the voltages at the terminals AD1 and AD2 of the AD converter 9 are obtained under the following condition.<Condition 1>
The sensor current at A/F=18 is 4 [mA], the element's admittance Ad at the element temperature 700° C. used for control is Ad=0.04 [1/Ω], and the shunt resistance Ra is 100 Ω.
In this case, the “A” voltage at the terminal AD2 during the sensor current detection (period a) is given as“A” Voltage=3.3 V+100  Ω×4 mA=3.7 V
On the other hand, the peak value B of the AD voltage during the element's admittance computation (period b) is given as“B” Voltage=“A” Voltage (3.7 V)+100 Ω×(0.2 V/(1/0.04 [1/Ω])=4.5 V
Since both the “A” and “B” voltages fall within the allowable voltage range of 0 V to 5 V for the conventional AD converter 9, no problem will arise as long as a sensor that matches condition 1 is used. However, if the A/F sensor characteristic changes as a result of a sensor design change, the “B” voltage may exceed the input upper limit value of 5 V of the AD converter. An example of this is shown below.<Condition 2>
The sensor current at A/F=18 is 4 [mA], as in the condition 1, but as a result of a sensor design change, the element's admittance at the element temperature 700° C. used for control has changed to 0.08 [1/Ω].
The “A” voltage at the terminal AD2 of the AD converter 9 in this case is obtained as follows.
First, during the sensor current detection (period a) the “A” voltage is given as“A” Voltage=3.3 V+100 Ω×4 mA=3.7 V
Next, during the element's admittance computation (period b) the “B” voltage is given as“B” Voltage=“A” Voltage (3.7 V)+100 Ω×(0.2 V/(1/0.08 [1/Ω])=5.3 Vwhich exceeds the input upper limit voltage value of the AD converter 9, and as a result, the voltage at the terminal AD2 is stuck at 5 V.
In this way, when the A/F sensor characteristic changes, the voltage at AD2 during the voltage sweeping may exceed the input upper limit voltage value of the converter, and the “B” voltage may become stuck at 5 V. If this happens, the element's admittance cannot be computed accurately. To address such a situation, it is usually practiced to reduce the shunt resistance Ra by one half to 50Ω thereby preventing the “B” voltage from being stuck at the upper limit value of the converter during the voltage sweeping.
If the shunt resistance Ra is reduced to 50Ω, however, the voltage at point A drops from 3.7 V to 3.5 V, and thus, the dynamic range decreases by one half. That is, the drawback is that the accuracy of sensor current detection drops to one half.
As described above, the A/F sensor current detection circuit of the prior art has the drawback that, when a sensor is used that has a characteristic different from the A/F sensor characteristic assumed at the time of designing the circuit, it becomes difficult to accurately detect the element's admittance while maintaining a high accuracy for the detection of the sensor current.